1. Field of the Invention
The present invention relates to monitoring of asynchronous transfer mode (ATM) networks, and more particularly, to in-service monitoring of end-to-end Quality of Service (QoS) in ATM networks.
2. Background of the Art
In an ATM network, a user can communicate information with another user through a connection, referred to as a virtual circuit, established in the network. The connection can carry voice, video, and data in fixed-size cells of 53-bytes, which include a 5-byte header and a 48 byte payload field. The connection can support a wide range of services defined by a set of quality of service (QoS) parameters.
The ATM Forum standard, Traffic Management Specification Version 4.0, ATM Forum/95-0013R10, February 1996, defines a set of QoS parameters that characterize the end-to-end performance of a connection in an ATM network. When establishing a connection through an ATM network, a user negotiates with the network the set of QoS parameters that characterize the connection. The particular set of QoS parameters depends on the type of service that the user requests from the network. These services include constant bit-rate (CBR), variable bit-rate (VBR), available bit-rate (ABR), and unspecified bit-rate (UBR).
Specifically, through a call admission control procedure, a user signals a request for a new connection in the network, and the network makes a decision to accept or reject the new connection request depending on the specified QoS and the available network resources. Generally, the network accepts a new connection if the network determines that the available network resources will be sufficient to satisfy the specified QoS. If the network accepts the new connection, the network implicitly agrees to a traffic contract that requires the network to sustain the specified QoS as long as the traffic on the connection conforms to the negotiated traffic contract.
The negotiated traffic contract underlaying a connection underscores the need for monitoring and verifying the actual QoS provided by the ATM network. Existing methods for monitoring the QoS in a connection require the users to measure the end-to-end performance of the connection by exchanging monitoring information in the ATM adaptation layer (AAL) or higher protocol layers. For example, users can exchange timing information to measure the end-to-end delay for transmission of cells in a connection.
The existing methods for monitoring the QoS in connections, however, have several disadvantages. First, ATM network switches, routers, and bridges can only access the cell header but not the cell payload. Furthermore, ATM networks cannot use the cell header for performance monitoring because the cell header has been designed to be short and simple to make fast cell switching or relaying possible. Finally, to reduce the processing in ATM switches, an ATM cell has been deliberately designed not to support a cell header field that would include performance monitoring information such as cell sequence numbers and time-stamps.
Second, when performing QoS monitoring, network management systems generally poll ATM switches for performance monitoring information. However, the performance monitoring information collected by the network management systems is generally specific to each switch, and does not include end-to-end QoS monitoring information.
Third, network operators commonly use specialized high-speed testing equipment to test and monitor connections in an ATM network. However, existing testing equipment for broadband networks, such as ATM networks, are typically expensive, complex to use, and intended more for controlled laboratory testing than large scale field application.
ITU-T Rec. 1.610, B-ISDN Operation and Maintenance Principles and Functions, Geneva, July 1995, defines a monitoring method, referred to as an operations and maintenance (OAM) method, for in-service performance monitoring in an ATM network. The OAM method inserts OAM cells between blocks of user generated cells. To use the OAM method, ATM switches must recognize the OAM cells and must relay them with the user cells. When a switch that functions as an end-point node receives an OAM cell, the switch processes the OAM cell and inserts its monitoring information in a new OAM cell, which the switch then transmits in the backward direction. Due in part to the additional processing and cell handling complexity associated with the OAM method, most existing ATM switches do not support these capabilities yet.
Furthermore, to reduce costs and integrate network management functions into a single hardware platform, others have attempted to implement monitoring systems for measuring the end-to-end QoS in a connection or a group of connections using a standard computer such as, a personal computer (PC). However, because of the multitasking operating environment of a PC, other tasks such as, user applications, screen savers, disk caching or even mouse movements, may contend with the monitoring system for CPU cycles, and thus, may restrict the accuracy and consistency of monitoring measurements that are time-sensitive.
For example, the monitoring system must time-stamp outgoing cells immediately prior to transmitting the outgoing cells into the network, and similarly, must time-stamp the incoming cells immediately after the monitoring system receives the incoming cells from the network. The resource sharing of the PC's operating system, however, may result in inaccuracies when the monitoring system calculates the time-stamps. Specifically, the monitoring system may not be able to calculate a time-stamp immediately upon arrival of an incoming cell, or may not be able to calculate a time-stamp immediately prior to the transmission of an outgoing cell, in part, because the PC's operating system may have already allocated the current CPU cycles to another task, causing the monitoring system to wait for free CPU cycles.
Thus, it is desirable to have a method and system for in-service monitoring of the end-to-end QoS in a connection or a group of connections in an ATM network without the above-mentioned disadvantages.